primary qualities of
matter which are necessarily involved in extension in space--as size,
figure, situation, divisibility, and mobility. From the combination of
these atoms all other existences are produced; fire, air, earth, and
water; sun, moon, and stars; plants, animals, and men; the soul itself
is an aggregation of round, moving atoms. And "motion, which is the
cause of the production of every thing, he calls _necessity_."[425]
Atoms are thus the only real existences; these, without any pre-existent
mind, or intelligence, were the original of all things.

[Footnote 424: Diogenes Laertius, "Lives," p. 395.]

[Footnote 425: Id, ib., p. 394.]

The psychological opinions of Democritus were as decidedly materialistic
as his physical theories. All knowledge is derived from sensation. It is
only by material impact that we can know the external world, and every
sense is, in reality, a kind of touch. Material images are being
continually thrown off from the surface of external objects which come
into actual contact with the organs of sense. The primary qualities of
matter, that is, those which are involved in extension in space, are the
only objects of real knowledge; the secondary qualities of matter, as
softness, hardness, sweetness, bitterness, and the like, are but
modifications of the human sensibilities. "The sweet exists only in
form--the bitter in form, hot in form, color in form; but in causal
reality only atoms and space exist. The sensible things which are
supposed by opinion to exist have no real existence, but atoms and space
alone exist."[426]

[Footnote 426: Lewes's "Biographical History of Philosophy," p. 96. The
words of Democritus, as reported by Sextus Empiricus.]

Thus by Democritus was laid the basis of a system of absolute
materialism, which was elaborated and completed by Epicurus, and has
been transmitted to our times. It has undergone some slight
modifications, adapting it to the progress of physical science; but it
is to-day substantially the theory of Democritus. In Democritus we have
the culmination of the mechanical theory of the Ionian or Physical
school. In physics and psychology it terminated in pure materialism. In
theology it ends in positive Atheism.

The fundamental error of all the philosophers of the physical school was
the assumption, tacitly or avowedly, that sense-perception is the only
source of knowledge. This was the fruitful source of all their erroneous
conclusions, the